Magnetic recording media generally comprise a magnetic layer coated onto at least one side of a nonmagnetizable substrate. For particulate magnetic recording media, the magnetic layer comprises a magnetic pigment dispersed in a polymeric binder. The magnetic layer may also include other components such as lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, nonmagnetic pigments, coating aids, and the like.
Some forms of magnetic recording media, such as magnetic recording tape, may also have a backside coating applied to the other side of the nonmagnetizable substrate in order to improve the durability, conductivity, and tracking characteristics of the media. The backside coating also includes a polymeric binder and other components such as nonmagnetic pigments (e.g., carbon black), lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, coating aids, and the like.
The polymeric binders of the magnetic layer and the backside coating are commonly derived from polymers which require curing in order to provide the magnetic recording media with appropriate physical and electromagnetic properties. To prepare such media, the components of the magnetic layer or the backside coating, as appropriate, are combined with a suitable solvent and thoroughly mixed to form a homogeneous dispersion. The resulting dispersion is then coated onto the nonmagnetizable substrate, after which the layer or coating is dried, calendered if desired, and then cured.
The polymeric binders of magnetic recording media are most commonly polymer blends comprising a hard component, i.e., a polymer with relatively high glass transition temperature and modulus, and a soft component, i.e. a polymer with relatively low glass transition temperature and modulus. In the past, polyurethane polymers have been widely used as the soft component.
Hard Resin Component
Copolymers based on vinyl chloride or vinylidene chloride have been widely used as the hard component of choice for use with polyurethanes, due to their miscibility and compatibility with polyurethanes and their relatively high glass transition temperatures, moduli, and hardness. For example, Japanese Publication No. JP61-26132 describes the use of vinyl chloride/vinyl acetate/vinyl alcohol co-polymers as a polymeric binder component in magnetic recording media.
Magnetic pigments tend to agglomerate and can be difficult to initially disperse in the polymeric binder, or to keep dispersed in the polymeric binder over time. Low molecular weight wetting agents, or dispersants, are often employed to facilitate such dispersion. For higher pigment loadings, i.e., the use of greater amounts by weight of magnetic pigment, greater amounts of wetting agent or dispersant may be required. This is not always desirable. Dispersants tend to plasticize binder systems and can decrease their modulus. Further, excess dispersant may exude from a cured binder system over time, leading to changes in the properties of the media as well as to contamination of a recording head or the like.
To help alleviate the problems associated with added low molecular weight dispersants or wetting agents, polymeric binders formed from "serf-wetting" polymers have been developed. "Self-wetting" polymers have dispersing groups pendant from the polymer backbone that help disperse the magnetic pigment. Representative examples of dispersing groups include quaternary ammonium, amine, heterocyclic moieties, salts or acids based on sulfate, salts or acids based on suffonate, salts or acids based on phosphate, salts or acids based on phosphonate, and salts or acids based on carboxyl, as well as mixtures thereof.
As a result of using self-wetting polymers, less low molecular weight dispersant or wetting agent, or even no low molecular weight dispersant or wetting agent, may be needed to disperse the magnetic and nonmagnetic (if any) pigments in the polymeric binder. Self-wetting vinyl chloride copolymers have been described. See, e.g., U.S. Pat. Nos. 5,139,892; 5,126,202; 5,098,783; 5,064,730; 5,028,676; 5,008,357; 4,861,683; 4,784,913; 4,770,941; and 4,244,987.
Vinyl chloride or vinylidene chloride copolymers, however, tend to degrade over time, releasing gaseous HCl which can change the properties of the media as well as corrode the recording head or the like. Accordingly, some investigators have described vinyl copolymers used in magnetic recording media, wherein the use of vinyl chloride and vinylidene chloride has been avoided. See, e.g., U.S. Pat. Nos. 5,098,783; 4,876,149; and 4,837,082; and Japanese Publication Nos. SHO 62-30162; SHO 54-84708; SHO 54-46519; and SHO 54-46518.
Soft Resin Component
A few polyurethanes have been proposed that contain phosphorous-based wetting functionalities, referred to herein as "P functional" polyurethanes. Generally, no evidence is apparent to suggest that any one type is to be preferred. The phosphorus functionality can be either pendant to the main polymer chain or an integral part of it. There are surprisingly few examples of the use of a phosphonate functionality for such purposes. Phosphorous-based functionalities are often preferred in conventional, low molecular weight dispersants. Examples of patents which disclose the use of phosphorus chemistry in self-wetting polyurethanes include JP 01-106324A, JP 02-053219A, U.S. Pat. No. 4,612,244 and U.S. Pat. No. 4,842,942.
Several patents disclose polyurethanes with both phosphorous-based and hydroxy functionalities at sites other than at chain ends. For example, JP 02-012613A describes P functional polyurethanes with tertiary hydroxy functionality at positions other than chain ends.
It appears unlikely, however, that a coating containing only polyurethane would provide all of the essential physical properties required. Thus, binders in magnetic media generally comprise both a soft resin and a hard resin. For example, YP 01-073523A and U.S. Pat. No. 4837082 discuss the need to find the correct balance of soft and hard resin components within the cross-linked binder matrix in order to maintain flexibility whilst ensuring that the matrix has the required tensile strength and modulus. The presence of a hard resin having a higher glass transition temperature ("Tg"), compared to the low Tg urethane, also provides for improved thermal stability.
JP 02-129217A, which discloses phosphorus-containing urethanes, contains no reference to the inclusion of hard resin binders, for example vinyl chloride or nitrocellulose materials. Other references that claim good dispersivity from the sole use of a self-wetting resin based on phosphorus-containing acid wetting groups can be found in US 5,037,934.
There are several disclosures of the use of binder blends containing self-wetting polymers. These can be divided into three groups; (a) self-wetting polyurethanes with "non-wetting" hard resins, Co) "non-wetting" polyurethanes with self-wetting hard resins and (c) the use of urethanes and hard resins which both contain polar wetting functionalities.
Examples from group (a) combine self-wetting urethanes, which disclose the inclusion of phosphorus functionality, with commercially available vinyl chloride based resins. JP 02-012613, for instance, discloses examples of self-wetting urethanes blended with vinyl chloride/vinyl acetate, vinyl chloride/vinyl propionate, thermoplastic polyurethanes, cellulose polymers, thermoplastic polyesters, epoxy resins, and phenolic resins. Other examples which use these blended binder materials can be found in JP 3224130A, JP 3224129A, JP 3224128A and U.S. Pat. No. 4,529,661.
With respect to group (b), U.S. Pat. No. 5,151,330 (corresponding to JP 3219417) describes the use of a binder blend that includes a polyurethane resin containing polycaprolactone units and a self-wetting vinyl chloride resin containing various polar groups including phosphonic and phosphoric acids and salts. Similar examples are described in JP 3041618 and U.S. Pat. No. 5,096,774.
With regard to group (c), several patents describe binder blends in which both materials (urethane and hard resin) contain polar groups for pigment dispersing. However, the choice of such polar groups is generally limited to acids and salts comprising either sulphur or phosphorus. For example, JP 3203020 teaches the use of two different resin types, vinyl chloride or cellulose derivative or polyester resin with a polyurethane or polyester resin, each containing a polar group from, SO.sub.3 M, COOM or PO(OM).sub.2, where M= H, alkali metal, or ammonium. Other patents which disclose such wetting functionalities include JP 3141018, U.S. Pat. No. 4,830,923 and U.S. Pat. No. 4,784,914.
At present then, most commercially available magnetic media is made using a binder system which includes a blend of a polyurethane resin and a vinyl chloride-containing copolymer. Because of the potential of degradation caused by dehydrochlorination of the vinyl chloride hard resin, it remains highly desirable to eliminate vinyl chloride from magnetic media. While binder systems based on the omission of vinyl chloride and the inclusion of wetting groups have provided some improvement, it is generally recognized that even further improvement in certain respects would be highly desirable. For instance, it would be particularly desirable to have a binder system that provides further improvement in a variety of performance characteristics, particularly including modulus, Tg, and smoothness.